Wind-turbine generating units having wind turbines and generators that are rotationally driven by the wind turbines are provided on wind turbine generators at the top of wind turbine towers in a manner capable of yaw rotation. The wind turbine generators are constructed such that the wind turbines receive wind force from the front and such that the wind-turbine generating unit yaws (turns in a substantially horizontal plane) with respect to the wind turbine tower in response to the wind direction.
In the wind turbine generators, yaw driving of the wind-turbine generating unit is carried out by securely disposing a ring gear on the tower, positioning a pinion that engages with the ring gear on the wind-turbine generating unit, and rotationally driving the pinion with a yaw motor (for example, see Patent Document 1).
Patent Document 1:    Japanese Unexamined Patent Application, Publication No. 2001-289149